1. Field of the Invention
This invention relates to a process including a shaft or deep boring in the earth, commonly known as wells, for the extraction of fluids from the earth. More particularly, this invention relates to a process for recovering hydrocarbon from a subterranean formation using a well or wells for injection and production and including heating steps.
2. Description of the Prior Art
In many areas of the world, there are large deposits of viscous petroleum. Examples of viscous petroleum deposits include the Athabasca and Peace River regions in Canada, the Jobo region in Venezuela and the Edna and Sisquoc regions in California. These deposits are generally called tar sand deposits due to the high viscosity of the hydrocarbon which they contain. These tar sands may extend for many miles and may occur in varying thickness of up to more than 300 feet. Although tar sands may lie at or near on the earth's surface, generally they are located under an overburden which ranges in thickness from a few feet to several thousand feet. The tar sands located at these depths constitute one of the world's largest presently known petroleum deposits.
The tar sands contain a viscous hydrocarbon material, which is commonly referred to as bitumen, in an amount which ranges from about 5 to about 20 percent by weight. This bitumen is usually immobile at typical reservoir temperatures. For example, at reservoir temperatures of about 48.degree. F, bitumen is immobile, having a viscosity frequently exceeding several thousand poises. At higher temperatures, such as temperatures exceeding 200.degree. F, the bitumen becomes mobile with a viscosity of less than 345 centipoises.
In situ heating is among the most promising methods for recovering bitumen from tar sands because there is no need to move the deposit and thermal energy can substantially reduce the bitumen viscosity. The thermal energy may be introduced to the tar sands in a variety of forms. For example, hot water, in situ combustion, and steam have been suggested to heat tar sands. Although each of these thermal energy agents may be used under certain conditions, steam is generally the most economical and efficient and is clearly the most widely employed thermal energy agent.
Thermal stimulation processes appear promising as one approach for introducing these thermal agents into a formation to facilitate flow and production of bitumen therefrom. In a typical steam stimulation process, steam is injected into a viscous hydrocarbon deposit by means of a well for a period of time after which the steam-saturated formation is allowed to soak for an additional interval prior to placing the well on production.
To accelerate the input of heat into the formations, it has been proposed to drill horizontally deviated wells or to drill lateral holes outwardly from a main borehole or tunnel. Examples of various thermal systems using horizontal wells are described in U.S. Pat. No. 1,634,236, Ranney; U.S. Pat. No. 1,816,260, Lee; 2,365,591, Ranney; 3,024,013, Rogers et al; 3,338,306, Cook; 3,960,213, Striegler et al; 3,986,557, Striegler et al; and, Canadian Pat. No. 481,151, Ranney. However, processes which use horizontal wells to recover bitumen from tar sand deposits are subject to several drawbacks.
One problem encountered with use of horizontal wells to recover bitumen is the difficulty of passing a heated fluid through the horizontal well. During well completion bitumen will sometimes drain into the well completion assembly. This bitumen may block fluid flow through substantial portions of the horizontal well and thereby decrease heating efficiency.
Another problem encountered with using horizontal wells for recovering bitumen by thermal processes is the difficulty of recovering bitumen which drains into the well. Conventional mechanically energized pumps or pneumatically energized displacement pumps are generally not satisfactory for recovering bitumen from horizontal wells. It has been proposed to use the formation pressure to move the bitumen through the horizontal section of the well and to lift the bitumen to the earth's surface. It is well known, for example, that wells which have been stimulated by "huff and puff" processes sometimes need no artificial lifting due to the hydrocarbon viscosity reduction and to the increased pressure resulting from steam injection. However, to economically recover fluids by this method, the viscosity of the production fluids must be kept relatively low. As bitumen is produced through conventional horizontal wells it has a tendency to cool and to increase in viscosity to the point where the formation pressure will no longer force it to the earth's surface. As a consequence, the efficiency of the steam stimulation process declines.
There is a substantially unfilled need for a thermal system using substantially horizontal wells to effectively recover bitumen from tar sand deposits.